To prevent serious injuries, especially at the cervical level (caused by what is known as “whiplash” and which can cause an entire series of injuries in the neck, vertebrae and spinal cord), in the event of an automobile accident, it is important that the occupants of the vehicle have the headrests placed in a correct position when the accident occurs. To that end, headrests conventionally have means allowing their adjustment, both in height and in closeness to the head of the user. However, in many cases, the position of the headrest is adjusted manually, therefore it depends on the intervention of the user. This implies that if the user does not correctly position the headrest, it is probable that he or she drives having it in a position which can involve serious injuries in the event of an accident. These problems are accentuated when the same vehicle is driven by people with different heights.
Therefore, it would be desirable for the headrest to be automatically positioned when the user occupies the corresponding seat. It could even be of interest for this positioning to be carried out continuously, being adapted to the changes of position of the user.
Some parameters which may be relevant for suitably placing a headrest in a vehicle are the following:                the vertical position of the headrest; it is usually considered that the upper edge of the headrest must be aligned (vertically) with the upper edge of the head of the user.        the distance between the headrest and the head of the user; this distance must be the suitable one to prevent an impact in the neck area.        
These distances (both in a vertical direction in a horizontal direction) must be within a range suitable for reducing the relative movement between the head and the trunk of the occupants and preventing the serious injuries mentioned above. A well-located headrest must first limit the longitudinal movement of the head and then its backward rotation.
In other words, the headrest must not be located excessively far from the head of the user. In some countries such as in the United States, according to regulations, the headrest must be in contact with the head of the user. For that reason it is important for the distance between the headrest and the head of the user (i.e., basically, the position in a horizontal direction of the headrest) to be adjustable or configurable according to the regulations in force, with the preferences of the user and/or with other criteria.
There are several publications which relate to devices for the adjustment in a vertical direction (i.e., in height) of the headrest:
GB-A-2320426 describes a system using sensors which can determine the position of the inner rear-view mirror and the position of an outer rear-view minor, to thus calculate the position of the eyes of the driver and, based on such position, to determine the vertical position of the headrest. This system represents certain drawbacks. For example, it requires sensors outside the headrest associated to the rear-view minors, whereby the system acquires a certain complexity in its implementation in the vehicle. The calculation is carried out approximately since the position of the minors is not directly related with the exact position of the eyes. Furthermore, the position of the eyes does not allow exactly determining the position of the upper end of the head and, therefore, the exact position which the headrest must occupy. Furthermore, this system only serves to adjust the headrest of the driver.
GB-A-2383530 describes a system making a calculation of the position of the head from a sensor arranged in the headliner of the vehicle or in the headrest. Based on this calculation it places the headrest in a certain vertical position. However, this system is not very accurate in the detection of the position of the head and, therefore, in the detection of the position of the upper end of the head.
FR-A-2810931 describes a device comprising a light emitter arranged, for example, in the rear-view minor, and a receiver housed inside the headrest to receive the beam of light from the emitter. This system is not very accurate since the detection of the upper edge of the head depends on the relative position between the position of the head and the light emitter, which can give rise to errors. Furthermore, the beam of light can be annoying since it can impinge on the eyes of the user. The system furthermore requires elements which are arranged outside the headrest, complicating the implementation of the system in a vehicle.
U.S. Pat. No. 6,402,195 describes a headrest incorporating two condensers, arranged at different heights such that the head of the user opposite the condensers acts as a dielectric producing an output signal in the condensers. By comparing the signals of the two condensers, the device can determine the optimal position of the headrest in a vertical position. However, this form of detection with condensers perhaps cannot determine the position of the upper edge of the head very accurately, but rather it will make estimate the vertical position of the head according to the closeness thereof to the headrest.
DE-A-10062511 describes a method for positioning a headrest based on the measurement of the distance of the rear part of the head to the headrest, comparing measured values with theoretical values by means of a calculation algorithm. This method is therefore based on a theoretical estimation of the position of the head.
U.S. Pat. No. 6,042,145 describes a headrest comprising an infrared sensor scanning the head of the user and calculating the vertical position of the headrest. This device is not very accurate either since it is based on calculating the position of the point closest to the headrest and, based on such position, calculating the position of the highest point of the head. In other words, the position of the highest point of the head is not measured but rather this position is determined indirectly and approximately. The closeness of the headrest is adjusted by moving the entire seat closer or further away. U.S. Pat. No. 5,848,661 describes a headrest and a back of a seat comprising multiple pressure sensors through which the system can determine (estimate) the position of the head and thus vertically adjust the headrest. The method does not seem very accurate and furthermore requires the head to be supported against the headrest.
JP-A-8-187139 describes a similar device using a single pressure sensor arranged in the headrest.
U.S. Pat. No. 4,645,233 describes several embodiments of a device for adjusting the height of the headrest. In one embodiment a light emitter arranged in the front part of the vehicle roof and a receiver arranged in the headrest are used. In another embodiment a sensor arranged in the roof just above the head of the user is used. In another embodiment there is a sensor arranged in the front part of the roof and which responds to the reflection of light from the eyes of the user. The possible use of a pyroelectric sensor detecting the heat emitted by the head of the user is also described. An embodiment is also described in which a light emitter-receiver is incorporated in the upper part of the headrest to detect the position of the upper end of the head.
All the documents mentioned above relate to systems which lack an adjustment of the headrest in the sense of closeness/separation with respect to the head, i.e., to systems which do not comprise an adjustment of the distance between the head of the user and the headrest, i.e., an adjustment in a horizontal direction of the headrest.
U.S. Pat. No. 6,088,640 describes a system which allows adjusting the position of the headrest in height (vertical adjustment) and in closeness. For the vertical adjustment, the headrest incorporates a sensor made up of an emitter and a receptor which can detect the position of the head with respect to the headrest (horizontal distance or closeness). By moving the headrest up and down, the system determines the position of the point at which the head is closest to the headrest. Based on this piece of data, the system calculates the position of the highest point of the head. To that end, it uses a series of theoretical patterns with which it compares the values obtained by the sensor. The patterns include situations in which the user can wear a hat, a scarf, long hair, etc. It carries out adjustment in closeness by contact, moving the headrest forwards until it meets a hard object (which is taken to be the head), using to that end a contact sensor. This system has the drawback that the adjustment in height is carried out based on an estimative method and the adjustment in closeness is carried out until the device contacts the head of the user, which may be valid for the United States regulations but may be uncomfortable (especially in view of the fact that many users do not want to have the headrest in continuous contact with the head) and, therefore, not very attractive for users in countries which do not require the head to be in continuous contact with the headrest.
There are also devices adjusting the position of the headrest based on different means. In this sense, JP-A-3-221005 describes how the headrest is moved by acting on switches, i.e., by the intervention of the user. EP-A-1270316 describes a system in which the headrest moves forwards automatically activated by the signal of an acceleration sensor which warns of an imminent impact. Devices of this type are common but have the drawback that they depend on a collision sensor and on the reaction capacity of the motors and mechanisms. It may be preferable to always have the headrest in the suitable position.